In the past, wax precipitation was conducted under conditions of low or no agitation. This procedure was followed since it was believed that precipitation under conditions of high agitation would result in the formation of fine wax particles which would clog the liquid-solid separators. The typical wax precipitation technique employed scraped surface chillers. In such a unit a waxy oil and a dewaxing solvent are premixed at a temperature sufficient to effect complete solution of the oil and wax. If necessary, the waxy oil is heated (either prior to or after additions of solvent) to insure complete solution of the wax contained therein.
The solution is then indirectly cooled at a uniformly slow cooling rate, e.g., 1.degree. to 5.degree. F./min., under conditions which avoid substantial agitation of the solution during precipitation of the wax. Because of fouling of the exchanger wall in the indirectly cooled heat exchangers due to wax deposition on the exchanger surface, scrapers are employed to remove the wax. However, because of the physical crushing of the wax crystals formed on the chiller wall by the action of the scrapers, nonuniform crystal growth occurs which results in slow filtration rates and large amounts of occluded oil in the wax.
The DILCHILL.RTM. (DILCHILL is a registered service mark of Exxon Research and Engineering Company) process was developed so as to overcome the inherent limitations and disadvantages of scraped surface chilling dewaxing. In the DILCHILL process, cooling is accomplished in a staged tower. The waxy oil is moved through the tower while cold solvent is injected along the tower directly into a plurality of the stages (either some or all of the stages have cold solvent injected into them). The cold solvent injection is accompanied by a high degree of agitation in at least a portion of the stages containing waxy oil and solvent so as to insure substantially instantaneous mixing of the cold solvent and waxy oil. Chilling is conducted to a temperature of between about 0.degree. and 50.degree. F. A substantial portion of the wax is precipitated from the waxy oil under these conditions of cold solvent injection and high agitation. The DILCHILL process is described in greater detail in U.S. Pat. No. 3,773,650, hereby incorporated by reference.
A modification of the DILCHILL process is presented in U.S. Pat. No. 3,775,288, also hereby incorporated by reference. In the modified DILCHILL process, cooling by means of cold solvent injection and high agitation is conducted to a temperature greater than the temperature at which the wax is separated from the oil, i.e., the wax separation temperature but generally less than about 40.degree. F. above said separation temperature and preferably less than about 35.degree. F. above said separation temperature, thereby precipitating at least a portion of the wax from the waxy oil. This oil-solvent-wax slurry is then withdrawn from the DILCHILL chilling zone and introduced into a second chilling zone wherein it is cooled to the wax separation temperature, thereby precipitating a further portion of the wax from the waxy oil. Cooling rates in this zone are in the range 5.degree.-20.degree. F./min.
This modification is practiced so as to avoid employing the large volumes of cold solvent which would otherwise by necessary to reduce the temperature of the oil-solvent-wax slurry all the way down to the wax separation temperature. In this embodiment, the second chilling zone may incorporate any conventional cooling process such as scraped surface chilling, autorefrigeration and the like; however, scraped surface chilling is preferred. In scraped surface chillers, the partially cooled oil-solvent-wax slurry is indirectly cooled to the wax separation temperature without the addition of more solvent. The scrapers are used to remove any wax which adheres to the walls of the chillers. A disadvantage of the scraped surface chiller in this embodiment is the same as that encountered when employing scraped surface chillers as the sole cooling unit. The scrapers physically crush the wax crystals formed on the chiller wall thereby reducing the wax filtration rates and increasing the amounts of occluded oil in the wax.
U.S. Pat. No. 4,140,620 to Paulett describes an incremental dilution dewaxing process wherein a lubricating oil stock, at a temperature above its cloud point is cooled in a cooling zone with vigorous agitation to a temperature below its cloud point and then further cooled with minimum agitation and incremental solvent addition to its final temperature, followed by filtration for the removal of wax. Rapid stirring is provided during the early part of the cooling period. The cooling zone is described as being a conventional double wall heat exchanger provided with means for agitating the oil during cooling by more rapid rotation of the scrapers. The base oil stock is diluted with solvent, during the initial agitated chilling. The major portion of the solvent is added to the system after the initial wax crystals have formed, i.e., after the temperature of the oil base stock, with or without dilution, has reached a temperature slightly below the cloud point of the waxy petroleum fraction. From the figure in the patent it is seen that the cooling zone comprises a double wall chiller wherein the waxy oil feed is introduced into the inner zone with cold filtrate supplied to the outer jacket of the chiller, with increased agitation being provided by increased rotational speed of the scrapers.
It is clear that the bulk of the solvent is added after the initial high agitation cooling and before or during the low or no agitation final cooling steps.